1. Field of the Invention
The present invention relates to a technique effectively applicable to an apparatus and a method for detecting from a picked-up image a specified object or a part of the object such as a man, an animal or any other physical object included in the particular image.
2. Description of the Background Art
A conventional technique is available for detecting a specified object or a part of the object such as a man, an animal or any other physical object included in a picked-up image. An example of this conventional technique is to detect a human face from a picked-up image (See Paul Viola, Michael Jones, “Robust Real-Time Object Detection” SECOND INTERNATIONAL WORKSHOP ON STATISTICAL AND COMPUTATIONAL THEORIES OF VISION-MODELING, LEARNING, COMPUTING, AND SAMPLING VANCOUVER, CANADA, Jul. 13, 2001 (hereinafter referred to as Non-patent Reference 1)).
In Non-patent Reference 1, a specified rectangle (hereinafter referred to as “the face judging rectangle”) to be processed is moved in an image thereby to judge whether a human face is included in the face judging rectangle of each destination (hereinafter referred to as “the intended area”). FIG. 14 is a diagram showing an example of the face judging rectangle (face judging rectangle P1). With reference to FIG. 14, the process of detecting a human face using the face judging rectangle P1 is explained.
The face judging rectangle P1 includes therein a plurality of other rectangles (hereinafter referred to as “the first rectangle”, “the second rectangle”) P2, P3. The first rectangle P2 and the second rectangle P3 are arranged at predetermined positions in the face judging rectangle P1. At least one first rectangle P2 and at least one second rectangle P3 are arranged in one face judging rectangle P1.
In the face detecting process, the feature of each area defined by the first rectangle P2 and the second rectangle P3 (hereinafter referred to as “the first feature area” and “the second feature area”) in each intended area is calculated. The feature of the first feature area or the second feature area indicates, for example, an average pixel value in the respective area.
Next, the difference between the feature La of the first feature area and the feature Lb of the second feature area is calculated. In accordance with whether this differential value is larger or not than a preset threshold value α, it is judged whether a human face is included in the intended area or not. The threshold value α is determined by learning using a sample image.
In the actual process, a plurality of patterns of the face judging rectangle are prepared, and the judgment is made for each pattern. Each pattern has a different number or position of the first rectangles P2 or the second rectangles P3. Based on each judgment result, it is finally judged whether the intended area includes a human face or not.
The aforementioned technique for detecting a human face by calculating the difference of the feature between the first feature area and the second feature area as described above is also disclosed in Japanese Unexamined Patent Publication No. 2000-123148 (Patent Reference 1).
The accuracy of detecting a human face in an image has been improved by employing a method using the face judging rectangle P1 as described above. Currently, however, the process of detecting a human face in an image is required to be executed in real time on a device such as a portable telephone having a machine specification not comparatively highly sophisticated. Therefore, an improved processing speed is also required. At the same time, the accuracy of human face detection in an image is still required.